The function of a high-intensity discharge (HID) electronic ballast is to supply ignition to the lamp for starting and then operating the lamp, such as a metal halide lamp. A metal halide lamp is a gas discharge lamp in which metal halides are enclosed, for example, in a quartz envelope.
To initiate its operation, a metal halide lamp demands a high ignition voltage. Once the lamp is ignited, the voltages falls to low voltage of the order of 20 V and the lamp it is then maintained for a short time (typically between 1-2 minutes) in so-called “current mode” where the current is constant and the voltage rises until the lamp reaches nominal power, whereafter the ballast serves to stabilize the power.
Prior art igniter circuits are known where an uncontrolled oscillator frequency is swept from a frequency that is less than the resonant frequency such that when it reaches resonance the voltage reaches maximum value and the lamp strikes. However, during this operation the frequency continues to rise and the voltage therefore falls.